CDMA communication system performance for a constellation of CubeSats around the Moon

Abstract

In this paper a communication system for CubeSats in formation that operate in the vicinity of the Moon is proposed. A CDMA system for the fleet of CubeSats in the vicinity of the moon to communicate with the Earth station is considered. This is an extension of our previous proposed CDMA system for a concept Constellation of CubeSats. In this paper the Doppler effects on CDMA communication system performance for a constellation of CubeSats around the Moon will be investigated. As an example we have estimated the maximum Doppler and Doppler rate profile of a SOLARA/SARA CubeSat scenario. We investigate the Effects of Doppler Shift/Rate on the CDMA system performance as a result of the CubeSat constellation orbiting in a halo orbit around Earth-Moon Lagrange Point L1. A detailed analysis and simulation of system in presence of Doppler frequency and unknown carrier phase will be performed. First we define the CDMA system for uplink and downlink between the vicinity of the Moon and Earth station. Link budgets will be provided both for uplink and downlink. Bandwidth limitations imposed by the spectral standards will be investigated for modulation formats. All system simulations are done using Simulink Matlab platform. For highly efficient nonlinear power amplifiers, we prefer to use a filtered offset QPSK with phase modulation, which is a CCSDS standard for constant envelope signaling. This allows us to use a nonlinear amplifier at CubeSat to operate at saturation point for the highest efficiency. Filtered offset QPSK with phase modulation is much more bandwidth efficient scheme. We demonstrate that this modulation format satisfies the two international spectral standards. For estimated and specified Doppler frequencies and Doppler rates we design frequency-tracking loops to track the Doppler frequency and Doppler rate in the presence of data with filtered offset QPSK with phase modulation. For carrier phase offsets a well-designed tracking loop is derived with specified loop bandwidth for the same modulation format. We use the CCSDS standard LDPC codes for space applications to meet the link budget margins.